Near-Earth asteroid 4179 Toutatis will be passing within 7 million kilometers of Earth on December 12. It's visited us several times before, with a close pass every four years in December. As near-Earth asteroids go, it's a good-sized one, an elongated and lumpy object about 2 by 2 by 4 kilometers in extent.

As with all encounters since 1992, it will be the target of an imaging campaign from some of the world's great radio telescopes, including those at Goldstone and Arecibo. In fact, the Goldstone observations have already started. At the Minor Planets Mailing List, Lance Benner posted that they expected to achieve resolutions of 7.5 meters per pixel, and hoped to achieve 3.75 as the asteroid gets very close, with very high signal-to-noise ratio. I'm looking forward to those close-approach images!

JPL / Courtesy Lance Benner

Radar image of asteroid 4179 Toutatis (beginning of 2012 approach)

Resolution = 18.75 m x 0.032 Hz. Weighted sum of 32 runs. Range increases down and Doppler frequency increases toward the right. Toutatis is an irregular and very elongated object. The group of bright pixels at the bottom right is an echo from a topographic feature that's barely visible along the radar terminator; it's not a satellite.

This particular close approach by Toutatis is extra-special, because Chang'E 2 (China's erstwhile lunar orbiter) is on its way to a flyby, with a close approach on December 13. According to radio astronomer Michael Busch, Chang'E 2 will fly within a few hundred kilometers of Toutatis.

Chang'E 2 produced beautiful photos of the Moon, but it will be a major challenge for it to obtain photos of Toutatis. Chang'E 2's camera, like most mapping cameras on orbiting spacecraft, is a pushbroom-style imager that is designed to take advantage of the spacecraft's predictable, steady orbital speed to sweep an array of pixels along the ground, with the ground a fixed and predictable distance away from the spacecraft. This asteroid encounter bears no resemblance to an orbital mapping mission. Chang'E 2 will be passing Toutatis at a high relative velocity of 11 kilometers per second, which means that the distance to the target will be changing very rapidly. In order for Chang'E 2 to get a photo, it will have to very carefully aim its narrow-angle imager in the correct direction and slew the spacecraft in order to scan the linear camera detector across the asteroid and therefore acquire an image. It's a process that I described when I posted some Mars Express images of Phobos last year.

It helps that the orbit of Toutatis is extremely precisely known (thanks to all those previous radar observations), but still, it will not be easy for Chang'E 2 to succeed. If it does succeed, it will obtain at most two images, one on approach and one on departure, with resolutions of a few tens of meters. This isn't any better than the radar resolution, but images would identify albedo patterns and could help disambiguate radar-derived models of Toutatis' shape. This is an extremely challenging thing for China to attempt, especially given that this is their first deep-space encounter. They only recently brought online a radio telescope of the kind you need to perform the necessary deep-space communication and spacecraft tracking and navigation. I hope that they do succeed, as it'd let me add one more object to my asteroids and comets poster!

Comments:

Gary Ray Rogers: 12/06/2012 01:05 CST

I was going to ask about radio telescopes and how they can take an image. But I decided to look it up and found your good explanation;
http://www.planetary.org/blogs/emily-lakdawalla/2011/3248.html
Thanks again for helping us understand Spaaaace Science.

Fred Thurber: 12/07/2012 11:28 CST

Does the Chinese space agency (CNSA?) have any statements about the mission to 4179 Toutatis? Or did we have to intuit this from the crafts trajectory? Why the secrecy?

Paul Cox: 12/07/2012 09:38 CST

We'll be broadcasting live images of Toutatis in a couple of free public shows on the night of the 11th/12th Dec.
I'll be setting up three of the slooh.com robotic telescopes to track the asteroid, including the Half Metre telescope. Details and timings of the shows are on the Slooh homepage.
The observatory is in the Canary Islands which is 4-7hrs ahead of USA time so the shows are at an ideal time for those who can't stay up late on a working day. We'll also broadcast a late show at the time of closest approach (2012-12-12T06:40UTC) with live images from Arizona, together with some popular objects using the Half Metre telescope at the Canary Islands Observatory.
There are also a couple of other NEOs making their closest approaches a few hours after Toutatis, so we'll probably track those too.

RanJones: 12/08/2012 08:30 CST

I'm with you, Emily. I hope the Chinese succeed! :-)

Jonathan McDowell: 12/11/2012 05:15 CST

Emily, above you say Chang'e encounter is Dec 13 but you cite http://www.planetary.org/blogs/emily-lakdawalla/2012/06150926-change2-update.html where you say Jan 6 - which is correct?

Emily Lakdawalla: 12/11/2012 06:39 CST

Jonathan, I linked to the wrong post. I should've linked to Bill Gray's guest post, which corrected the flyby date that had been given in Chinese media to December 13. I've corrected the link in the blog entry. Thanks for pointing that out!